Telephone or like signaling system



Sept. 3, i 946. I c. GILLINGS VEFAL 21 TELEPHONE OR LIKE SIGNALINGSYSTEM Filed Sept. 18, 1943 8 Sheets- Sheet 1 mvzn-rons YP CHARLESGILLINGS CHARLES Emuw BEALE TESEO mo DAN'IE TERRONI ATTORNEY Sept. 3,1946. c. GILLINGS ET AL 2,407,150

TELEPHONE OR. LIKE SIGNALING SiSTEM Filed Sept. 18; 1945 s Sheets-Sheet2 s m w m CHARLES GILLINGS CHARLES EDMUND BEALE TESEO BRUNO DANTETERRW gVTR CSZ Sept. 3, 1946.

I c. GILLINGS ET AL TELEPHONE OR LIKE SIGNALING SYSTEM Filed Sept. 18,1943 I I cndl' a Sheets-Sheet s c'nd5 INVENTORS CHARLES sums CHARLESsouuuo new:

' TESEO aamoomrsmmm EQRNEY s P c. GILLINGS E AL 2,407,150

TELEPHOiiE OR "LIKE 'SIGNALIEJG 'ISYSTEM FildSebt. lsfie 4s 'ashet-sheet 4 v II CTITI.

" INVENTORS CHARLES GILLINGS CHARLES EDMUND BEALE TESEO BRUNO DANTETERRONI Sept. 3, 1946- c. G|LLINGS ET AL 2,407,150

TELEPHONE 0R LIKE SIGNALING SYSTEM Filed Sept. 18, 1945 8 Sheets Sheet 5AAAL ' irz YA VSF lfivem'ons CHARLES eumes cums Eouum as:

TESEO BRUM DANTE TERRONI Sept. 3; 1946. c. GILLINGS ETAL 2,407,150 7TELEPHONE OR LIKE SIGNALING SYSTEM Filed Sept. 18, 1943 8 Sheets-Sheet 6WR-ZR F1 6 INVENTORS CHARLES GILLINGS CHARLES EDMUND BEALE TESEO BRLMDANTE TERRMI ATTORNEY Sept. 3, 1946. c. GILLINGS' ET AL TELEPHONE ORLIKE SIGNALING SYSTEM Filed Sept. 18, 1943 8 Sheets-Sheet 7 mvemonsCHARLES GILLINGS CHARLES EMU BEALE TESEO BRUNO DANTE TERROM ATTORNEY.

Sept. 3, 1946.

C. GILLINGS ET AL TELEPHONE 0R LIKE SIGNALING SYSTEM Filed Sept. 18,1945 8 Sheets-Sheet 8 il llllll llll Ill I6 I 1 l I l l I l l Ill lllll|llll lL I 'INVENTORS CHARLES GILLINGS cums eouuno am: n-zsso anuuo wasmam Patented Sept. 3, 1946 UNITEDSTATES PATENT oFricE TTELEPHONE OR'LIKESIGNALING SYSTEM Charles Gillings, Charles Edmund Beale, and Teseo BrunoDante Terroni, Liverpool, England, assignors, by mesne assignments, toAutomatic Electric Laboratories, Inc., a corporation of DelawareApplication September 18, 1943, Serial No. 502,884

- In Great Britain October 15, 1942 7 claims. (o1. 179 27 'May 31, 1943,now Patent No. 2,376,352, dated May 22, 1945.

According to this method of V. F. signaling, the numerical directcurrent impulses received at the sending end of the trunk are convertedinto voice frequency coded signals which are transmitted over the trunkand re-converted at the terminal end into' numerical direct currentimpulses. Furthermore, each voice frequency coded signal is preceded bya prefix signal which is the same for all codes and serves the purposeoftesting the incoming equipment and also switching any echo suppressor inthe route into the correct direction of transmission preparatory to thesending of the coded signal which is characteristic of the D. C. trainof impulses to be transmitted.

This method of signaling gives a good degree of speech immunity in thatby reason of their length and combination the signals used rarely ifever occur in speech, In fact the degree of 4 immunity against falseoperation is so gOOd with this method of signaling that the use ofso-called stopper valves which have hitherto been generally used in V.F. systems to prevent the exten- .coming. backwardly over the line tothe calling end and accordingly, therefore, any toxic signals receivedduring the setting up of the connection were received quite freely. Whenthe connection was completely set up and the called partly answered, ananswer signal was sent backwardly over the trunk to bring about theremoval of the stopper .valve at the sending end so that conversationcould take place in both directions of transmission. I

Although the use of astopper valve provides a good degree of protectionagainst disturbing currents generated at the sending end interferingwith the V. F. receivers, its presence in the line is nevertheless'undesirablein certainv circumstanceaparticularly-in the case ofnon-metered calls. Generally speaking this type of call is routedthrough to an.operator's position and the terminating apparatus issuchthat it does not send back an. answering signal so that the operator maytalk to the calling party and give him i the information requiredwithout metering the call against him. When, however, such a call isoriginated over] a V. F. trunk including a stopper-valve, itis necessaryto arrange the terminating apparatus to send back an answersigjnalin-order to bring about the removal of the stopper valve toenable conversation to take place. As will be appreciated this isparticularly undesirable in the case of automatic calls because a theanswer signal brings about automatic metering of the call. against thecalling party and it is therefore necessary in such circumstances forthe operator tomake out a credit slip for such calls in order thatrtheyshall be deducted from the calling subscribers account. From anoperating point of view such arrangements are undesirable and it is oneof the advantages of the present invention that they have been avoided.

In order to give a gooddegree of speech irnmunity for supervisorysignalswhich are transmitted over the trunk after the connection has been setup, it is arranged according to the inventionthat the apparatus measuressuch signals as regards their frequency, their duration and, theirspacing. The sequence is such that it is connected in circuit withthetrunk line leading to'a following exchange, said blocking device byintroducing considerable attenuation into the speaking circuitpreventing the code signal from actuatingflapparatus at said .followingexchange.

The invention will be better understood from M the following descriptionof one method of carrying it into effect, reference being bad to theaccompanying drawings comprising Figs. 1-9. Of

these drawings, Figs. 1-4 show the circuit arrangements of asleeve-controlled outgoing relay set, Figs. 5 and 6 show the circuitarrangements of an incoming relay set, Figs. '7 and 8 form a trunkingdiagram of various combinations of trunk routes to Which the inventionmaybe usefully applied while Fig. 9 show an alternative circuitarrangement including certain modifications which are necessary if theincoming relay set is required to respond to 'busy flash signals.

Before the detailed circuit .operations are described, a descriptionwill be given of .the general method of working with reference to thetrunking diagram, Figs. '7 and -8. 'In this diagram exchanges A-H areshown interlinked by V. F. and D. C. trunks. Lik pieces .of apparatus inthe various exchanges have been given like designations, for example, MBis manual board, O/G is outgoing relay set, 1/0 is incoming relay set,VF is voice frequency receiver, AA BS is auto-to-auto relay set, whileVF. 'TRK and D. C. TRK are voice frequency and direct currentinter-exchange trunks respectively.

Assuming that the operator at exchange B plugs into the jack at themanual board MB leading to 'a free outgoing relay set O/G, hersupervisory 'lampwill flicker and the 'outgoingrelay set will send a 100milli-seconds (m. s.) X voice frequencypulse forward over the trunk tothe incoming relay set 1/0 at exchange C tobring about the seizure of anassociated first selector. The operator now operates the dialling keywhereupon the supervisory lamp 'is extinguished and when she dials therequired 'subscribers number, the impulses are received and. stored onuniselectors in the outgoing relay -set. Each train of impulses receivedat the outgoing relay set is re-transmitted over the trunk in coded formmade up of a combination of four voicefrequenc'ies, hereinafter referredto as W, X, Y and Z. Each coded digit transmitted is preceded by apreparatory signal comprising all four frequencies, W, X, Y and Z whichare effective "in the incoming relay set 170 to prepare the voicefrequency receiver thereat for operation. The prepare pulse also takescontrol of any echo suppressors in the route to turn them .into thecorrect direction for transmitting. Conveniently the 'W','X Y,Zfrequencies used are of 900, 750, 600 and 500 cycles respectively.

The coded digit comprising not more than three of the frequencies W to Zis decoded at the incoming relay set and converted into a marking on thebank of a uniselector which counts the sending out of a correspondingtrain of loop impulses to set the automatic switches in exchange C.

.After an interval which is measured in the outgoing relay set and issufiicient to allow for the setting up and hunting time of the automaticswitch in exchange C, the second prepare and code signals aretransmitted and are dealt with similarly.

When all digits have been dealt with and the switching equipment atexchange C has been set to connect with the required subscriber, thecalling operator will receive ringing tone, busy tone, or iNU tone todenote the state of the called line.

When the called subscriber answers, repeated Y pulses of .140 m. s.duration spaced 36.0 m. s. apart are sent back from th incoming relayset in exchange C to bring about the extinguishing of thesupervisorylamp inthe operators cord circuit at exchange B. Vhen this is done, anacknowledgment signal is transmitted back from the outgoing relay setcomprising a single Q40 m. s. pulse of X frequency which serves toterminate the sending of the repeated Y pulses from the incoming relayset. Conversation now takes place and when the called party clears,repeated Y pulses similar to those forming the answer signal aretransmitted from the incoming relay set back to the outgoing relay setwhere they bring about the lighting of the operators supervisory lamp.When the operator clears in response to the clear signal, the outgoingrelay set sends forward to the incoming relay set a clear signalcomprising a two-seconds X pulse followed by a 300 m. s. Y pulse whichcauses the incoming relay set to clear down and release the train ofautomatic switches at exchange C.

In case the operator has to clear in face of ringing, busy or NU tone,the clear signal under these circumstances is lengthened to asix-seconds X 'pulse followed by a 300 m. 5. Y pulse. The object of thelong X pulse is to ensure that the clear signal will get through in faceof NU tone which is interrupted at the source for one second after thetone has :been transmitted for 'five seconds.

'The proposed voice frequency digit code is as follows:

The proposed voice frequency nal code is as follows:

supervisory sig- As willbe seen from the trunking arrangements ofexchange C, the outgoing relay set may be taken into use from any one ofthree sources, namely, manual board, automatic local subscriber, orincoming relay set from a distant exchange. In the case of seizure froman automatic local subscriber, the operations are substantially similarto those already described for seizure from the manual board, exceptthat resistance battery is connected to the sleeve of the manual boardjack to busy the outgoing relay set against seizure from this source.

In caset'he outgoing relay set in exchange C'is taken into use via anincoming relay set in the same exchange, a battery signal over bothlines is extended backwardly from the outgoing relay set to the incomingrelay set to cause the incoming rela set to switch the lines straightthrough and cut out ,of circuit the decoding and retransmittingapparatus. At the same time in the outgoing relay set the lines areextended by Way of a repeating coil bridge to the incoming .relay set inexchange D and the storing and code sending apparatus in the outgoingrelay set of exchange C is 7 cut outof circuit, Code signaling thereforetakes I5 place directly from the outgoing relay set of exchange'B to theincoming relay set of exchange D.

In case the call from exchange C isto be routed over a D. C. trunk toexchange E, access will be had from the incoming selector to thelineside of the auto-to-auto relay set A--A RS, thus cutting this relay setout of circuit. lhis arrangement is desirable to avoid two repetitionsof the impulses in the same exchange. The incoming rela set extendsearth forwardly to hold the automatic switches in exchange C.

If now the call to exchange E is extended over a VF trunk to exchange G,signals passed between th outgoing relay set in exchange E and theincoming relay set in exchange C bring about the setting up of straightthrough connections and the disabling of the coding and impulsingapparatus. Signaling therefore takes place directly from the outgoingrelay set in exchange B to the incoming relay set in exchange G so thatthe conditions become similar to the VF to VF case already described.

In case the connection from exchange E is extended over a D. C. trunk toexchange F and then over a VF trunk to exchange H, a different set ofconditionsarises because an auto-to-auto relay set at exchange E is nowinterposed in the trunk route. It is therefore now impossible to signalby means of the simplex battery connection between the outgoing relayset of exchange F and the incoming relay set of exchange 0 to causethese sets to switch over to straight-through connections. On the otherhand, unless means are provided to prevent it, the VF signals from theoutgoing relay set of exchange B can find a circuit 'by way of thecondenser bridge in the auto-toauto relay set of exchange E to theincoming relay set of exchange H and this would bring about thegeneration of two trains of impulses for each train of impulsestransmitted from the outgoing relay set in exchange B. To prevent thisthe incoming relay sets are provided with blocking devices in the linecircuit which function almost instantaneously upon reception of the VFprepare signal precedin each code and thus prevent the code signalsreaching the incoming'relay set at a dis- &

tant exchange such as exchange H. Thus the sequence of impulsing insofaras setting up of automatic switches in exchange H is concerned is asfollows. VF signaling between exchanges B and C; conversion at theincoming relay set of exchange C to D. C. impulses which extend by wayof exchange E and the auto-to-auto relay set threat to outgoing relayset of exchange F. Reconversion takes place at exchange F to VF signalswhich extend to exchange H and are there changed back again to D. C.impulses to set up the automatic switch train in exchange H.

Referring now to the detailed operations involved in setting up aconnection over the equipment shown, when the operator plugs into jackJCK, Fig. 1, if the cord circuit speaking key is thrown relay M isoperated over the sleeve circuit and in turn operates the relief relayMM, Fig. 4. Relay MM at contacts mm I, Fig. 1, connects flicker earth tothe upper low resistance winding of relay M to flash the lamp in theoperator's circuit to provide an indication that she is connected to adialling trunk, and at contacts mm2, Fig. 4, connects earth to the Pconductor incoming from selector levels to guard the relay set againstseizure over this path and also operates relay SR. Relay SR- at contactssrl causes the switch TS to self-drive from its home position toposition l where relay CO is operated over the oil-normal bank TSI andrelay FXY is operated over bank.

T84. Relay CO atcontacts e03 disconnects relay SR and also at contactscol and 002, Fig. 2

working the jack points Ill and H are strapped together as shown. Forbothway trunk working, however, this strapping is removed and the jackpoints Ill and ii are cross-connected to the termination in the incomingrelay set. Relay FXY in operating, at contacts fang l brings up relay CSand at contacts fats/2 causes the switch TS toselfdrive over its bankTS3 to the third position. During the period that the switch isself-driving over the two strapped contacts, X frequency is applied totransformer VXF via contacts fc2, fxyl and cal and terminatin resistanceVTR and then extends via contacts csl and 082 and contacts col and 002to the trunk line to form a seizing signal to the incoming relay set atthe distant exchange. With switch TS in position 3, relay FXY releasesto terminate the X pulse after a period of the order of 100 m. s. and atcontacts h2g3 operates relay FC which looks and at contacts I03 causesthe switch TS to step to position 4-from which it drives to position 6over bank T and interrupted earth connected to conductor 13. Relay FXY,in releasing, at contacts fan/4 releases relay CS.

When the operator throws the dialling key, battery is connected to bothlines and relay RR, Fig. 1, is operated over its upper winding, while atthe same time a high resistance isintroduced into the sleeve circuit toextinguishthe calling supervisory lamp. Relay RR at contacts rrloperates relay A which in turn'o-perates relay B, Fig. 3,1;0 preparecertain impulsing circuits and operate relay BR, Fig. 4. Relay BR atcontacts brl drops relay RR and at contacts brl and M2 connects relay Ato the tip and ring lines extending to the operators dialling circuitand also at contacts 1213 disconnects the flicker earth circuit andconnects up relay KR into the sleeve circuit;

this relay however will not operate owing to the high resistance alreadyintroduced therein. When the dial isturned off-normal, balanced batteryon the lines is replaced by a loop through the dial impulse springs andrelay A continues to hold. When the dial is released, relay A respondsto the impulses which it repeats at contacts al, Fig. 3, via relay C tothe driving magnet DSAM of the first digit storing switch DSA so thatwipers of this switch are stepped to a corresponding position. Relays Band C hold operated during the impulsing and the latter at contacts 02,Fig. 4, operates relay OS to prepare the VF signaling circuit atcontacts csl and 082, Fig. 2. Relay C also at contacts cl, Fig. '3,energises the driving magnet DDM of the digit distributor switch DD, butas this switch is of the reverse drive type its wipers are not moved atthis time.

At the end of the first series of impulses, relay A remains energisedand relay C releases after its slow period and at contacts 0! completesa circuit over bank DSAI, Fig. 3, for operating relays CNA and CNR inseries. If the digit dialled is 5 or less, relay CNA operates over itsleft-hand winding and as a result a short interdigital pause is providedbetween the retransmitted digits as will appear subsequently. Relay CNRat contacts cnr2 advances the: sending control switch SC from its homeposition to position I where relay ST is operated over its two windingsin series. Relay approximately 160 ms. .holds relay CS and at contactss3, Fig. 2, con- -ST thereupon at :contacts stl short-circuitsitsleft-hand high resistance winding to self-drive the sending controlswitch SC to position 4, at contacts 'st2, Fig. 4, maintains relay CSand at contacts Sl38i3 causes a prepare pulse of WXYZ frequency to beconnected via the transformer VXF to the trunk line to the distantexchange. When the sending control switch SC reaches contact 4, relay STreleases after its slow period thereby disconnecting the WXYZ preparepulse and at contacts stl operates relay S 'over its'two windings inseries with magnet SCM.

The duration of the prepare pulse is thus meas- .ured by the timetakenfor the switch SC to step from contact i to contact '4 plus theslow release period of relay ST which ,gives a total figure of Relay Sat contacts s5 nectsthe code pulse to the line, this pulse beingdependent for its composition on the setting of the switch DSA as fullydescribed in the previously mentioned specification so that it is thuscharacteristic of the digit dialled. Relay S also at contact 81short-circuits its high resistance left-hand winding to self-drive theswitch SC to contact :9 where relay Z operates over its high resistancelower winding and at contacts 24 disconnects the code pulse from thetrunk line. The

duration of the code pulse is measured by the time taken for switch SCto self-drive from contact 4 to contact 9 which is approximately 100 ms.Relay Z holds relays CNA and CNR at contacts 2] and at contacts 22causes the switch DSA to self-idrive to its home position where relayZreleases and at contacts 22 causes the switch SC also to drive to itshome position, while at contacts 23 relay-CS is released.

If in the meantime the operator has dialled the second digit on toswitch DSB, (notshown) relay CNR will be maintained operated in serieswith a relay CNB, .not shown, and accordingly when the switch SC comesinto its home position it will again self-drive to contact -I whererelay ST will re-operate to-send out the prepare pulse of WXYZ frequencyfollowed by the code pulse which .is picked up from the setting of theWipers of the switch DSB which is assumed to besimilar to the switch DSAalready described.

It should be explained here that if, as :has been assumed, the digitdialled was 5, the interdigital pause provided between the two codedigits transmitted to line will be measured :by 20 self-driving steps ofthe switch DSA, say 400 m.s., plus 16 self-driving steps of switch SC,say 320 m.s..giving a total of 720 ms. approximately. During this periodfive machine-generated pulses are delivered at the distant exchange tothe incoming selector after which hunting must take place to select anidle trunk in the level selected. While the selector is hunting, theincoming decoding relays are released to receive the second prepare andcode signals which will require a time period of 160 plus 100 ms. givinga total of 260 ms. which may be added to the 720 m.s. already mentionedto give a total of approximately 980 ms. for the complete interdigitalpause during which the incoming selector must be set to the requiredlevel and perform its hunting operation.

If, however, the first digit dialled had been 6 to 0, a longerinterdigital pause would be required to enable the incoming selector toperform its sequence of operations and this is derived as follows. RelayCNA under these conditions would :be operated over its right-handwinding in series with relay CNR so that when relay S operated,

bank and wiper DSAI.

set the cyclic function.

relay EP would operate and lock to earth over Accordingly, when theswitch DSA reaches its home position, thus measuring 01f part of theinterdigital pause, relay EP will hold during the slow period due to itsshortcircuited upper winding and at contacts 6102 will cause the switchDSA to make another revolution. During this revolution relay EP releasesbut relay Z holds to bring 'about a substantial increase in theinterdigitalpause. 'For example, if the digit O had been dialled, the.interdigital pause would be made up as follows: fourteen steps of theswitch DSA plusan additional revolution making 39 steps all gives aperiod of approximately 780 ms. To-this must be added sixteen steps ofthe switch SC say 320 ms. plus a further 260 ms. representing thetransmission time of the prepare and code pulses of the second digitgiving a total interdigital pause of approximately 1,360 ms. The objectof vthis extra pause period is to ensure that the distant selector hassufficient time to perform its level-selecting function plus itstrunk-hunting function before the next decoded digit is transmitted tothe succeeding switch.

When all stored digits have been transmitted in code, relay CNR finallyreleases and at contacts cnrZ opens the kick-on circuit for the sendingcontrol switch SC, while at contacts curl, Fig. 4, it brings up relayBS, Fig. 2, which has no functionat this stage.

Although four digit storing switchesDSA-DSD have been provided, it ispossible that in practice a lesser number may be found satisfactorydepending upon the speed at which storing and retransmitting in code canbe accomplished and upon the number of digits to be dialled. As shownthe digit storing switches DSADSD are taken into use in cyclicrepetition, that is tosa-y, when the fourth digit has been stored on theswitch DSD, the switch DSA which will by this time be normal is againtaken into use to store the fifth digit and so the storage cycle repeatsitself. With the ONA-CND digit storing circuit shown in Fig. 3, however,it is possible that if the fifth digit dialled is received before allthe first four digits have been sent out in coded V. F. form, the DSAswitch, on which the fifth digit is received, will take precedence andso will up- This trouble may be simply overcome by providing, in the CNRrelay individual battery feeding circuits to the three relays CN-A-CNC,break contacts of relay CND which are paralleled by make contacts ofeach relay concerned. As regards relay CND this will remain directlyconnected to relay CNR as shown in Fig. 3. With these arrangements theoperation in turn of the relays CNACNC is not interfered with and onoperating they will lock independently of relay CND, but as soon asrelay CND is operated in response to the fourth digit, none of thepreceding relays can be operated in response to a fifth and subsequentdigits. This condition obtains until the fourth digit has been sent outin coded V. F. form after which the battery feeding circuits of thethree relays CNA- CNC are recompleted. If by this time the fifth digithas been dialled, the 'DSA switch will have been already set accordinglyand relay CNA will now re-operate to provide for the sending out of thisdigit. Similar remarks would apply to the sixth and seventhdigits, whilethe eighth digit would function-on relay CND in the same manner as forthe fourth digit. By this means it is contemplated that up to ten digitscould be safely handled by the four digit switches DSADSD.

When the operator restores her dialling key at the endof dialling, thecurrent in the'sleeve circuit is increased and the battery applied tothe tip and ring conductors is removed shortly afterwards. Relay KR,Fig. l, thereupon operates and locks up at contacts lcrl while atcontact lcrZ it releases relay A. Relays B and BR in turn restore aftertheir slow periods and the former at contacts b3 advances thedigitdistributors-witch DD to its mid positionpreparatory to the operation ofrelay AA at a later period. Relay BR at contacts br3 connects theearthed low resistance lower winding of relay RR into the sleeve circuitand though relay RR receives insufiicient currentto operate thesupervisory lamp again glows under this condition. Relays M, MM, KR, CO,.FC and BS remain operated and the line condition is now such that theoperator may listen to tones and if necessary speak without thenecessity for a calledparty answer signalg but no supervision is given.

If the operator restores her cord circuit speaking key the sleevecircuit current is increased. to a value sufficient to operate relay RRand this relay at contacts rr2, Fig. 1, maintains a suitable V. F,termination on the trunk line, in place of the operator's telephoneset.

When the called; party answers, the incoming relay set at the distantexchange transmits 140 m. s. of Y frequency over the trunk followed bya, space period of 360 m. s. and this signal is repeated untilacknowledged by the transmission of an X signal from the outgoing relayset. Upon the reception of the Y frequency, relay Y operates in the YFreceiver VFR shown schematically as a dotted rectangle in Fig. 2 andopens the circuit to relay BS which releases after a slow period of80-110 m. s. To release relay BS therefore the Y frequency must beapplied for a sufficient period and. when relay BS has released, relaysMS and MT are operated inturn and hold for the remainder of'the Y pulse.When the Y pulse is finished, relay BS re-operatesand at contacts bslopens the circuit for relay MS which has a release timeof 300-400 m. s.Relay MT has a release time of 200-300 m. s. If the break period is ofthe correct duration, namely 360 In. s.,then thisperiod plus'the 80-110m. s. release time of relay BS, which releases in response to he next Ypulse, will be. suflic-ient to cause relay MS to release but not relayMT. Hence on the reception of the next Y pulse the "subscriber answerrelay SAwill be operated over contacts sy2, sal, sb2, ms], mtl, syl,bsl, ac5, yl and cm! to earth. At the end of the Y pulse, or when relayMT releases, relay SB which has been short-circuited hitherto operatesin series with relay SA and locks.

Relay SB operates relay AA, Fig. 3, in the selfdriving circuit oftheswitch DD and relay AA at contacts aa2, Fig. 4, reoperates relay CSand at contacts aal, Fig. 2, applies an X pulse acknowledgment signal tothe outgoing trunk. The duration of this acknowledgment signal ismeasured by twelve steps of the switch, DD plus the release time of therelay AA which releases when the DD switch reaches the home position andwhich in turn releases relay CS. When the Xpulse is received at thedistant incoming relay set, the transmission of the Y pulses ceases andrelay BS remains operated.

' It should be mentioned that relay SA at contacts sa3, Fig. 1, connectsbattery via resistances YP and YQ to the ring of the jack for throughsupervision and alsoatcontacts safi, Fig. 1, removes the short-circuitby way of the lower winding of relay RR from the high resistancewinddistant exchange transmits Y pulses of the same typegas forthecalled party answer signal.

At the outgoing end relays M, MM, KR, CO, FC, BS, SA and SB are heldoperated and the switch TS is in position 6. Accordingly the first m. s.pulse of Y frequency releases relay BS and operates relays MS and MT asalready described. During the following 360 m. s. break period relay BSis operated and releases relay MS but not relay MT so that when the nextY pulse is received and relay BS again releases, during the slow releaseperiod of relay MT relay SY operates over contacts $172 and at contactssyl completes a locking circuit for itself-for the remainder of the Ypulse. Relay SY also at contact sy3 operates relay SZ and at contactssyZ releases relay SA which at contacts sa3, Fig. 1, disconnects batteryfromthering of the jack for backward supervision, and at contacts saBrestores earth via the low resistance winding of relay RR to the jacksleeve circuit to re-light the operators calling supervisory lamp. g

As long as the Y pulses persist, relay SY remains held because itsrelease time is 400-450 m. s., i. e. longer than the 360 m. s.breakperiod and consequently if the called subscriber should againremove his receiver, the Y pulses will cease and after its slow periodrelay SY releases and disconnects relay SZ. During the slow releaseperiod of relay SZ relay SA re-operates and locks in series with relaySB to restore the speaking conditions.

When the operator clears by removing the plug from the .jack in responseto the clear signal, relays M and M in the sleeve circuit release andthe latter releases relay KB. The release of relay MM at contacts mm2,Fig. 4, steps the switch TS from position 6 to position 1 by way of bankTS3, whereupon relay BR operates in series with relay SR over bank TS4.Relay SR on operating at contacts. sr3 re-operates relay CS and theserelays together bring about the application of an X frequency pulseforwardly over the trunkby way of contacts sr2, csl and 082. Since relaySB is operated, the switch TS continues to self-drive from position I toposition I5 via bank TS3 and contacts sb4 and then steps by means ofinterruptedearth on lead 13 from position I 5 to position [9 via bankTS5. Relay FXY is then uperated over bank 'I'Sfl, contact l9, and servesto disconnect the X frequency at contacts .fxyl, Fig. 2, after a periodof application of two seconds and at the'same contacts to connect up Yfrequency to the trunk. When the wiper leaves position ill of bank TS I,relays SR and BR are causedto release after their slow periods and onthe release of the latter, earth is extended over bank TS3 to step theswitch to position 20. Relay FXY then releases after its slow period andthus terminates the Y signal after a period of application of 300 m. s.and also releases relay CS. Switch TS then steps from position 20 toposition 23. via bank TS3 and contacts acl, and then slow -steps fromearth on lead l3 over contacts 23 and 24 via bank TS5 to reach its homeposition where relays CO, FC and SB release. During the forward;application of Y frequency relay BS will have released and at the endofthe signal relays SY and SZ release in turn. Relay QQ disconnects thebusy condition from the sleeve of the jack at contacts 005, Fig. 1, and;as earth is also removed from the P conductor leading to the selectorlevels when the TS switch reaches its home position the relay set isfree for further use.

Consideration must now be given to the condition when the operatorclears before receiving the answering signal or in face of any tone(ringing, busy or NU) or on a non-metered call. Any of these conditionsare characterised bythe nonoperated condition of relay SB so that whenthe operator withdraws the plug and releases relays M and MM, the switchTS is caused to slow step from position I to position l9 byway ofinterrupted earth supplied over lead [3. Accordingly, therefore, the Xpulse which precedes the Y pulse is lengthened from 2 to 6 seconds,after which it is followed by the normal 300 m. s. Y pulse.

The object of the long X pulse under these con- 13 ditiOns is to ensurethat the clear signal will be received at the distant incoming end when,for example, NU tone is being transmitted backwardly. The NU tone istransmitted for five seconds and disconnected for one second and this :7

arrangement ensures that the clear signal which is being extendedforwardly shall be able to break inand register at the incomingrelayset.

When the outgoing relay set is taken into use over the negative,positive and P trunks extending from selector levels, the call may havebeen originatedeither by a subscriber on the same exchange or via anincoming V. F; relay set from a distant exchange, say 13 In the firstcase the subscribers dialled impulses are converted by the switcheDSA-DSD into VF prepare and code pulses; while in the second case, wherethrough working is required, the outgoing relay set merel transmits theX seizing signal-"to the idstant' incoming VF-relayset afterwhichfurther prepare and code pulses are. transmitted directly through theoutgoing relay set to the distant incoining VF= relay set withoutmodification.

' In the case when the outgoing relay-set is taken into use fromasubscriber in the same exchange, earth extended forward overthe privateconductor? operates relay-SR, Fig. 4, which causes the switchTStoself-drive over bank TS3 to the first position where'it-operatesrelaysCS and Relay C0 operates relay BS; and drops relay SR, whilerelayFXYextends a 100 m. s. X seizing-pulse forwardly over the trunk tothe distant incoming relay set. Relay FXY also causes switch TS toself-drive-to-position 3 where relayFXY isdisconnected and after itsslowperiod-releases toterminate the X pulse. Relay FC is then operate-dand-locks up and serves'to step TStopositionA from which it slow stepsfrom interrupted earth on conductor; l3; to positionfi Relay FC alsobrings about-the operation of relay A, Fig. 1, over the callingsubscriber s loop-and relay A in turn brings up relaysB- and-BRtoprepare the circuitforthe reception of thedialled impulses. From thispoint thecall proceeds as for an operator call alreadydescribed; thatis-to say, relay-A responds to-thedialled-impulses which are stored-viathe digit distributor switch-DD on the coding switches DSA--DSD.Subsequently under the control ofthe sending 'sw-itchSC the codeddigits-aretra-nsmitted toline in. theform of prepare and code pulsesandare received on decoding apparatus at the incoming relay set in thedistant exchange. Relay CO under these, conditions busies the manualboard jack by. connecting. resistance battery to the sleeve-cir 1 2cuit. When the called partyanswer signal is received in the form ofrepeated 140 m. s. Y? pulses as already described; relays Ms and MTcheck the length and spacing of the pulsesand ifthese are correctallowrelays SA and SB to operate. Relay SA at contacts saA and sa5, Fig.1, reverses the A relay connections towards the call-- ing partyfor;supervision, while relay SB operatesrelay AA to cause an X pulseacknowledgment signal to be transmitted forwardly to the distantexchange. This signal lasts fora period measuredby the homing time ofthe switch DD-via thelow resistance relay plus the slow release periodof relay AA which is disconnected when switch DD arrives at its homeposition. The reception of the X" pulse acknowledgment signal at thedistant exchange causes the backward Y" answer signal tobe disconnectedand conversation may now takeplace.

In the case of a call via selector levels from an incoming VF relay set,earth is connected toboth the negative andpositive lines and serves tooperate relay AG; Fig. 1, while the normal forward earth on the Pconductor operates relay-- SR. Relay'AQ at contacts ac2 and ac3'connectsthe negative andpositive lines from the selector level through to theoutgoing-trunk linewhile relay SRsteps switch- TS from the home positionto contact I where relays CO, CS and FXY areoperated-a Relay-COoperates-BS, drops SRand' disconnects the trunk line termination TER.Relay FXY extends a 100 m. s. Xseizing pulse for-. ward overthe trunk tothe distant exchange and steps the switch Ts to contact 3'. In thisposition relay- FXY is disconnected and when it re-- leases after-itsslow period the X seizing pulse is terminated. Sincerelay AC isoperated,switch TS self-drives-- to position 20 via bank TSZ and remains; thereuntil the end of the call so that as it leaves position l while relayFXY is still'operated, relay F6 is not operated. The incoming-VFprepa-re and code pulses are thus extended directly throughtheoutgoing; relay set to the distant exchange; Relay CQ busies thejack byconnecting battery to the sleeve circuit, whileswitch TSat bankTSdholdsearth'on the P wire to busy the outgoing; relay set against seizureoventh'e selectorlevels. When-the connection is setup and thecalledparty-answers, repeated Y pulses are extended from'the' distant exchangedirectly through theoutgoing-relay set andare; acknowledgedby-a 440m; s.X acknowledgment signal extendingfrom-the callingend of the connectiondirectly through the outgoing relay set.- It-will benoticedthat the Yrelay of the-VF receiver' VFR isoperated from the Y pulse but as relayAC is operated atthis-time theresponse of relays'MS, MT. SA and SB isprevented by contacts1ac5, Fig. 2. When the calling end of theconnectioniscleared; earth isremoved from thenegative and positivelinesthus releasing relay AC which allows switch TS to self-driveovercontacts 20-23 viabank 'rs3- after which it a slowsteps to its homeposition.

If the operator inadvertently withdraws the plug while the calledpartyis still 'connected and possibly is speaking at rthistime, switch TSwill; run homefromposition 6 to extend the clear signal of two seconds'Xfollowed by 300 In, s;,Y'

and release the equipment in the outgoing, relayv set.- If however thecalled-party is, still talking at this time; any echo suppressor in theconnection will be held; against theclearing; signal which may =therefore fail to get throughto the distant end of therconnectionz It istherefore comprising as usual 140 m. s. of Y frequency followed by 360m. s. space period. The first Y pulse operates relays MS and MT andduring the first 360 m. s. break period relay MS releases so that at thecommencement of the next 140 m. s. Ypulse relay SA is operated and atcontacts 3012, Fig. 4, causes the switch TS to step from its homeposition to contact I. Relay SB is operated and locked when the Y pulseisfinished and from then on the switch TS makes a complete revolutionduring which the clearing signal comprising a two-second X pulsefollowed by a 300 m. s. Y pulse is transmitted to the trunk line torelease the distant equipment. It should be mentioned that during therevolution of the switch TS the X seizing pulse is again transmittedwhile the switch is passing over the early contacts of the bank, butunder these circumstances this signal produces no switching function atthe distant exchange.

Referring now to the circuit changes produced at the incoming relay set,Figs. and 6, when the 100 m. s. X seizing pulse is received over thetrunk line from the outgoing relay set, relay IX, not shown but assumedto be located in the valve receiver IVliRyoperates and in turn brings upthe relief relay XR. Relay XR at contacts m3 operates relay K whichlocks upat contacts kl, at contacts k2 operates relay PY, Fig. 6, and atcontacts k4 extends earth forwardly over the P conductor to the incomingselector to busy this switch. At the end of the X seizing pulse, relayIX in the valve receiver releases and in turn drops relay XR which atcontacts :rr4 operates relay NN. Relay NN at contacts nnfi furtherearths the P conductor extending to the incoming selector, atcontactsnnl operates relay KK over the negative line in series with onewinding of the A relay in the incoming selector and at'contacts nn3operates relay GX, Fig. 5. Relay GX at contacts 9034, Fig. 6, connectsbattery by way of a high resistance YF to the centre point of therepeating coil which finds a circuit to earth at the centre point of thetransformer which connects the valve receiver IVFR to line. A smallcurrent is thus causedto flow by way of various relay contactsconnecting with the line circuit which are thus wetted to preventcoherer trouble. No further operations take place at this stage and theincoming selector is now ready to receive the train of impulses.

" When the outgoing relay set transmits the 160 m. saprepare pulse ofWXYZ frequency as already described, relays IW, IX, IY and IZ operate inthe V. F. receiver and in turn bring up their relief relays WR, XR, YRand ZR. Over the series connected contacts :01'4, yrd, 213 and wr3 relayCA is now operated, whereupon the botliway line termination comprisingresistances YB, YC, YJ, YD and YE and condenser QA which normallyimposes a loss of the order of 3 db, is converted into a combined lineterminationand infinite attenuation loss pad which prevents the passageof signals so that the line is blocked at this point. The purpose ofthis arrangement will be described later in connection withtandeincalls.

-' The prepare pulse, as already described, has

a duration of approximately 160 m. s. and is followed immediatelyby thecode pulse which comprises frequencies W, X, Y and Z in suitablecombinations having a duration of'lOO m. s. In the example in questionthe digit 5 comprising the frequencies W and Z is assumed to have beentransmitted so that relays IX and IY are released and in turn drop theirrelief relays XR and YR. The short-circuit is thus removed from relay CBwhich immediately operates from the earthed contacts M12 in series withrelay CA, andat contacts c174 and 0191 looks relays WR and ZRcorresponding to the code which has been transmitted. Relays WR to ZRhave a pyramid chain of contacts (not shown)v connected to the bank SS2of the sender switch SS, and since the code relays WR and ZR areoperated at this time, earth is connected to the sixth contact ofthe'bank SS2 in order to terminate the sending after five impulses havebeen transmitted to the incoming selector as will appear subsequent: 1y.Relay CB also at contacts c225 releases relay GK and operates relays PCand P, the latter looking over its own contacts. Relay PC at contactspcI operates relay CC which at contacts ccI prepares a circuit for relayIG which operates when the constantly driven impulse springs 66%M nextopen. Relay IG at contacts z'g2 drops relay K and at contacts igIextends the impulse springs 66%M to the driving magnet SSM of the senderswitch. Relay IG also at contacts ig2 transfers the forward, holdingloop extending to the selector to the impulse springs 33%M which areclosed at this time, since they are out of phase with the magnet impulsesprings 56%M'which have just opened to bring about the operation ofrelay IG. Accordingly, the sending sWitchSS now steps its wipers untilthey encounter the marking earth connected to the sixth contact of bankSS2 over contacts of the coding relays WR and ZR, while at the same timethe impulse springs 33 %M transmit five impulses to the incomingselector as described in the previously mentioned specification. Relay45 SZ operates when wiper SS2 comes into its sixth position and locksover contacts seI to the homing. bank SSI. Relay SZ also at contacts s22short-circuits the 33%M springs to terminate the impulses to theselector and at contacts 823 50 releases relays CA and CB which havebeen held in series. Relay CB at contacts cbt drops relayIG which atcon-tacts ig4 causes the sending switch SS to.,-home, while the holdingcircuit for the selector is again transferred to relay KK 55 whichre-operates. Relay CB also re-operates relay GX and releases relays WRand ZR as Well as relay PC after its slow period which is followed bythe release of relay CC also after its slow period. Relays K, PY, NN,KK, GX and P 60 remain held.

After a pause long enough to permit the incoming selector to hunt overthe selected level, which pause is determined in the outgoing relay setas already described, the next prepare and.

65 code pulse is transmitted and the operation is repeated until all theimpulse trains have been transmitted. It will be appreciated that theprepare pulse of WXYZ frequency ensures that any echo suppressors in theconnection are switched 70 into the correct direction and provides atest that all the responding relays in the incoming V. F. receiver areoperating satisfactorily. If this is not so it will be impossible for acomplete connection to be set up and there is no danger of 75 connectionbeing made to the wrong party.

The connection is thus fully set up and the called party is rung. Whenthe called party answers, battery is reversed over the forward trunk sothat relay KK releases and relay EE operates. Relay EE at contacts eeZbrings up relay IR which at contacts irZ-ir4 alters the connection tothe loss-pad to render it inefiective and at contacts irl drops relay K.Relay K opens the circuit of relay PY and during the slow release periodof the latter, relay MA is operated and locks up over contacts mal andgx'l. Relay MA at contacts WW3 and mad, Fig. 5., connects up thesecondary winding of the transformer VSF to the backward trunk and atcontacts ma2 connects relay FY, Fig. 6, to conductor 14 which extends tointerrupted earth having a periodicity of 140 m. s. on and 360 m. s.off. Relay FY therefore pulses to the interrupted earth and extendsbackward over the trunk repeated 140 m. s. Y pulses with 360 m. s.spacing.

When this signal has been accepted at the outgoing relay set, the latterreturns a 440 m. s. X pulse acknowledgment signal which operates relayIX in the valve receiver and in turn the relief relay XR. Relay XR atcontacts m3 opens the circuit of relay GX and operates relay GY. Re layGX' releases after its slow period and at contacts grl drops relay MAwhich connects the line through for conversation and disconnects therepeated Y signal. Relay GY in operating cuts the earth pulse connectionto relay FY. Since the repeated Y signal is measured at the outgoingrelay set both for length and spacing, it will be appreciated that thisgives the desired degree of irmnunity against false Y signals which maybe generated by speech or line noise. Similarly, the X acknowledgmentsignal is measured against the release time of relay GX (300-400 m. s.)to provide the required degree of immunity against spurious Xfrequencies on the line. At the conclusion of the acknowledgment signal,relay XR releases, relay G-X re-operates and relay GY releases.

When the called party clears, battery is again reversed over the forwardline circuit so that relay KK re-operates and relay EE releases. RelayEE at contacts eeZ drops relay IR to re-introduce the line terminationand also operates relay MA. Relay MA connects interrupted earth to relayFY which pulses to send back to the outgoing relay set the clear signalcomprising 140 m. s. Y pulses with 360 m. s. spacing. As alreadyexplained, in the outgoing relay set this signal results in the releaseof relay SA which disconnects battery from the ring of the outgoing jackand lights the calling supervisory lamp at the operators position.Relays NN, KK, GX, P and MA remain held,-while relay FY is slowlypulsing;

When the operator clears, the outgoing relay set sends a clear signalcomprising a two-seconds X pulse followed by a 300 m. s. Y pulse; The Xpulse operates relays 1X, XR and GY in the incoming relay set and thelatter opens the circuit for relay F'Y to terminate the transmission ofthe called party clear signal. Relay XR also opens the circuit of relayGX sufficiently long for it to release whereupon it drops relay MA andoperates relay CC over contacts 9333. When the X pulse is finished,relays IX, XR and GY release and relay CC commences to release slowly.The 300 m. s. Y pulse now follows and serves to operate relays IY, YRand CR Relay CR locks to the earthed contacts 172' and transfers thehold ing circuit for relay NN to the Y pulse and also connects earth tothe P conductor extending for- Wardly to the automatic switches. Whenthe Y pulse is finished, relay NN releases and drops relays P and Kwhich open the holding circuit for relay CR, The forward holding loop isopened and the earth on the P conductor is removed on the release ofrelays N and CR thus initiating the release of the operated automaticswitches, and the incoming relay set is now free for further use.

In regard to tones which may be encountered during the setting up of thecall such as ringing, busy and NH, these are passed back along the trunkline in the normal manner except that the line termination which is incircuit under these conditions imposes a 3 db loss but this does notseriously reduce the tone level. The NU tone is broken at the source forone second in every six to allow the release signal to obtain control ofany echo suppressors which may be in circuit and thus get through to itsdestination. As explained above if the release signal is sent, beforethe called party has answered, it comprises 6 seconds of X frequencyfollowed by 300 m. s. of Y frequency. This long signal insures a certainbreak-in during the I second silent period of the NU tone.

Considering now the operation of the incoming relay set under theseconditions, since the called party has not answered, relay K will stillbe operated and hence the X signal operates relay PC which brings uprelay CC and from this point the release procedure is identical withthat previously described for the normal operator clear.

In case the incoming selector switches through to a short-circuitedtrunk, arrangements have to be made to prevent this short-circuitmasking the VF receiver and so preventing the incoming relay set fromaccepting the release signal of 6 seconds X followed by 300 m. s. Y. Inthe face of a short-circuit from the selector, neither of relays KK andEE is operated so that on the release of relays CB, PC and CC in turnrelay SC operates over contacts nus, 1001, 003, kkl, eei and p4 andlocks up at contacts .901. At contacts s02 and s03, Fig. 6, relay SCdisconnects the forward trunk leading to the short-circuit and underthese conditions the line termination is maintained since relay IR isnot operated and the clearing signals can be satisfactorily receivedfrom the outgoing relay set. I

In the case of tandem calls over two VF trunks in series, for instancefrom exchange B to ex change D, Figs. 7' and 8, the following operationstake place. The digits dialled cause the selector to seize an outgoingrelay set to another VF channel so that battery is returned from the ACrelay in this outgoing relay set over the negative and positive lines tooperate both relays EE and KK. Relay EE operates relay IR as alreadydescribed in connection with. the called subscriber answer condition,and relay IR releases relay K. As relay K is also operated at this time,however, relay PY is held operated and no circuit is completed tooperate relay MA and initiate the sending of the repeated Y answersignal backwardly over the trunk. With relay IR operated the loss padand line termination are removed at contacts ir2-ir4 while any circuitfor relay CB is disconnected at contacts 2'15' so that the following VF'signals pass directly through the incoming relay set without anyconversion or repetition. Under these conditions before the in-' comingrelay set can be releasedr it must receive a long X pulse to drop relayGX followed at the proper interval by a short Y pulse to operate enoughto bring about the-release of relay GX.

Furthermore, the 440mm; X pulse acknowledgment signal which is sent fromthe originating end in response to the called party answer signal willbring about the release of relay GX, but as this is not followed by a Ysignal thetemporary release of relay GX brings about no material changein the condition of the incoming relay. set. When the release signalcomprising along X pulse followed by a short Y. ulse is received, theincoming relay set releases as already described.

In the case of tandem calls over VF and D. C; trunks in series, forinstancefrom exchange B to exchange E, when theD. C. outlet is'seized inthe second exchange (C), access i's'had to the outgoing side'of thenormal auto-to-auto repeater AA RS, Fig. 7, which is thus cutout ofcircuit in order to avoid two impulse repetitions in the same exchange.The incoming relay set therefore extends earth forwardly over the Pconductor to holdthe selector train in the second exchange (C) and alsoaccepts code signals and transmits forward loop impulses to set up theautomatic switch train at the third exchange (E). When the called partyconnected to the thirdexchange answers, battery is reversed over theline tooperate relay BE in the incoming relay set at the second exchange(C) which thereupon sends repeated Y pulses back to the first exchange(B) as already described. Thefurther operation and release of theincoming relay set are as already described.

In the case'of tandem calls over VF, D. C. and VF trunks inseries forinstance from exchange B to exchange G, when connection is made with theoutgoing relay set in the third exchange (E), battery over both lines isextended backwardly over the trunk to operate relays'KK and EE in theincoming relay set at the second exchange (C) which thereupon cuts outits repeating apparatus and provides a straight-through pair of trunk asalready described. Accordingly, VF signals from the outgoing relay setinthe first exchange (B) now pass directly without repetition to theincoming relay set at the fourth exchange (G) which converts them to D.C. im-

In the case of tandem calls over VF, D. 0., D. C.

and VF trunks in series, for instance from exchange B to exchange H, adifferent set of conditions arises in that an auto-to-auto repeaterwillbe taken into use at the third exchange (E) which prevents battery on,both lines from the outgoing relay set in the fourth exchange (F)finding a circuit back to theincoming relay set in the second exchange(C) to cause this to provide a straight through pair of lines as wasthe. case in the VFD. C.-VF connection. In the absence of any meanstoprevent it therefore, there is a danger'that the V.'F.code signals fromthe originating exchange may be received both at the incoming relay setin the second exchange and the incoming relay set in the fifth exchange(H) and this would result in two train of impulses being delivered foreach digit. This explains the necessity for the provision of theinfinite attenuation loss pad in the incoming relay set.

Considering now the circuit operations, it will be remembered that whenthe VF prepare impulse is received at the incoming relay set at thesecond exchange (C), relay CA operates almost immediately and providesan infinite attenuation loss pad which in effect-short-circuits thetrunk across a resistance bridge to prevent the passage of V. F. signalsbeyond the incoming relay set. However, durin the operate time of relayCA at the incoming relay set in the second exchange (C), relay CA in theincoming relay set at the fifth exchange (H) may operate but will atonce release when the CA relay at the first incoming relay set hasoperated and inserted the loss pad in the trunk. It will be recalledthat relays CA and CB lock in series during the reception of the codepulse and while the loop impulses are being transmitted so that neitherthe prepare nor the code VF signals are transmitted to the incomingrelay set in the fifth exchange.

It will be appreciated that the desired block-.

i In the case of non-metered calls which are usually of the enquirytypesuch as service, interception, enquiry or speaking to B operators atmanual exchanges, it will be appreciated that with the circuitarrangements as described conversation can take place immediatelywithout the extension of an answer signal and therefore the automaticmetering of non-metered calls avoided. It will be seen that a 3 db losspad remains in the connection-under these conditions owing to theabsence of any'battery reversal which would operate relays EE and IR butthis does not unduly interfere with speech and furthermore if more thanone incoming relay set is included in the connection the loss pad isremoved in all incoming relay sets except the one in the terminatingexchange where no battery is returned over the negative and positivelines to effect the removal of the pad, and consequently the overallextra loss never exceeds 3 db on this type of call.

Referring now to Fig. 9, this shows the modifications necessary to theincoming relay set to instead of negative, relay TH is operated via themetal rectifier MBA and locks up. Relay IR; is also operated from relayEE and with all four relays operated the line termination and loss padis disabled and a straight through connection is provided.

In other circumstances where the incoming relay set is the terminatingone in the connection, relay KK will be operated while the call is beingset up. If a busy line is encountered, busy tone is returned asexplained and if this is accompanied by busy flash batteryon thepositive line relay EE is also operated. Since however, relay TH is notoperated under these conditions since the battery in question is anegative battery, relay IR in operating does not disable the linetermination and loss pad, so that balanced conditions are maintained tothe line amplifier which is prevented from singing during thebusyflashcondition.

What we claim as new and desire to secure by Letters Patent is: 1,. In atelephone system wherein a connection is established to, a calledstation by automatic switches controlled over a voice frequencysignaling trunk including means responsive to a disconnect at thecalling end to. transmit a forward signal to release said switches, asource of impulses comprising alternating current ofdetermined voicefrequency interrupted at predetermined intervals accessible to thecalled end of said trunk, means in said trunk responsive to a disconnectat the called station to connect said source of impulses to the trunk asa clearing signal whereby the calling station disconnects and theforward signal is transmitted, and means responsive to said forwardsignal to disconnect the clearing signals from the trunk before the saidswitch release is initiated.

2; In a telephone system wherein a connection is extended over a trunkin which the functional operations including superyisory and, releasingQ 'LS l Q 031 8 3 0 2 are cQ ll d y s snals r a plurality of sources ofalternating currents of different frequencies, means in said connectionto transmit; impulses of said currents of deterin d: f equen y dur tionand spa n over d trunk in accordance with said functionaloperations'controlled thereover, a signal comprising impulses one ofsaid frequencies repeated at a determined rate to control thesupervisory functional operation, a second signal comprising anotherfrequency of determined duration, means in said trunk to transmit saidsecond signal to stop said supervisory signal and means in saidconnection to transmit a subsequent pulse of said; first signal within adetermined spacing, after said stoppage signal is effective to controlthe functional operation of releasing said connection.

3. In a telephone system, a connection includinga trunk line over whichvoice frequency signals are transmitted to establish a connection and inwhich a clearing signal of voice frequency is normally sent tothecalling end when thecalled station hangs up and a release si al of Voicefrequency is then transmitted from the calling end to release theconnection, the provision of means responsive to a release signaltransmitted from the calling end before a. clearing signal. is receivedfor causing the transmission of the clearing signal to act as a releasesignal to release the connection.

i. In a telephone system, a plurality of voice frequency responsiveequipments each associated with a trunk line and each normallyresponsive to received voice frequency coded signals to translate thesame into direct current impulses to operate automatic switches toextend a connection, means in eachyequipment operated only in case asecond trunk is seized having such equipment therein in a tandemconnection for preventing the translation of received voice frequencysignals in the first equipment and causing the transmission of thereceived signals to the second seized trunk without translation, saidmeansresponsive only to a flow of current over th two speakingconductors in parallel.

5. In a telephone system, a trunk line, means for transmitting over saidline a plurality of coded signals each'comprising impulses of aplurality of different voice frequency sources and a varying number ofsaid frequencies in accordance with received signals and other controlsignals of varying number of impulses of determined frequency durationand spacing in accordance with other received signals, a plurality ofreceivers connected successively to said line and responsive to areceived signal for translating it to a signal corresponding to saidreceived. signal, a blocking meanscontrolled from each of said receiversto prevent the further passage of said received signal current over saidline to the successively connected receivers, means in each receivereffective on certain of said coded signals for translating said signaland operating said blocking device to prevent said other receivers fromeffectively completing said particular code translation and means ineach receiver responsive to other received signals to complete thecorresponding translation without operating said blocking means.

6. In a telephone system, a trunk line including voice frequencyresponsive equipment, means for extending av connection thereto and fortransmitting coded digit signals of voice frequency currents thereto toextend a. connection thereover and for transmitting other coded voicefrequency signals thereto, a blocking device in said trunk, each digitsignal including a portion effective to include the blocking device inthe trunk to block further progress of voice frequencies over theconnection, and each other signal passing over the connection withoutblocking.

7. In a telephone system, a voice frequency signaling trunk terminatedon a responsive repeater including means for transmitting and receivingover said trunk a signal to seize said repeater and another signal torelease said repeater, a. switch operating line controlled from saidrepeater, means responsive to the seizure of the repeater to connectsaid line to said trunk, said repeater responsive to other voicefrequency Signals received over the trunk to control a connection, andto repeat said release signal thereover, holding equipment in saidrepeater controlled over said connected line to prevent a false releaseof said trunk before said line is disconnected, means effective in theevent of a short circuit on said line whereby said holding equipment andsaid other signals are disabled, means insaid repeater effective onlywhen said equipment is disabled to disconnectsaid line from said trunkto permit said release signal to be effective to release said repeater.

CHARLES GILLINGS.

CHARLES: EDMUND BEALE. TESEO BRUNO DAN'I'E TERRONI.

